AbstractB lymphopoiesis arrests in rabbits by 4 months of age. To identify molecules that contribute to this arrest, cDNA–representational difference analysis on BM stromal cells from young and adult rabbits showed that expression of Postn that encodes for the extracellular matrix protein periostin dramatically reduced with age. Postn–small interfering RNA OP9 cells lost their capacity to support B-cell development from rabbit or murine BM cells, and reexpression of periostin restored this potential, indicating an in vitro requirement for periostin in B lymphopoiesis. In our system, we determined that periostin deficiency leads to increased cell death and decreased proliferation of B-lineage progenitors. Further, RGD peptide inhibition of periostin/αvβ3 interaction resulted in a marked decrease in B lymphopoiesis in vitro. Microarray analysis of the Postn–small interfering RNA OP9 cells showed decreased expression of key B-lymphopoietic factors, including IL-7 and CXCL12. In vivo, unidentified molecule(s) probably compensate periostin loss because Postn−/− mice had normal numbers of B-cell progenitors in BM. We conclude that the decline in periostin expression in adult rabbit BM does not solely explain the arrest of B lymphopoiesis. However, the interaction of periostin with αvβ3 on lymphoid progenitors probably provides both proliferative and survival signals for cells in the B-cell development pathway.